Phase behavior of LiClO4-doped poly(ε-caprolactone)-b-poly(ethylene oxide) hybrids in the presence of competitive interactions

The phase behavior of a series of LiClO4-doped poly(ε-caprolactone)-b-poly(ethylene oxide) (PCL-b-PEO) was studied as a function of PEO volume fraction (fPEO), doping ratio (r) and temperature (T). It is found that the morphology of the hybrids changes from disordered structure (DIS) to hexagonally packed cylindrical (HEX) structure and then to lamellar (LAM) structure as the volume fraction of the PEO/salt phase (fPEO/salt) increases at fPEO/salt < 0.5. Order-order transitions are observed upon heating some hybrids. An approximate phase diagram of the PCL-b-PEO/LiClO4 hybrids with fPEO/salt < 0.5 was constructed in terms of fPEO/salt and the segregation strength (χeffN). As compared with the phase diagram of the weakly segregated diblock copolymers, the phase diagram of the hybrids has two features: the boundaries of the LAM and HEX structures shifts to lower fPEO/salt and body-centered cubic spherical (BCC) structure is not observed for the samples studied. This can be attributed to the weaker ability of the salt inducing microphase separation at low fPEO and the conformational change of the PEO block induced by the salt. Some unexpected phase behaviors were observed for the hybrids with fPEO/salt > 0.5, including the hexagonally perforated layers (HPL) to LAM transition upon heating the same hybrid and HEX to gyroid (GYR) transition with the increase of doping ratio at the same temperature. These unexpected phase behaviors are qualitatively interpreted based on the competitive association of the PCL block with Li+ ions at elevated temperatures and higher doping ratios, which leads to re-distribution of the Li+ ions in different phases and the inconsistency between the calculated fPEO/salt and the real volume fraction of the PEO/salt phase.